It is well appreciated that one of the major problems facing road authorities and local councils is the maintenance and replacement of a large population of aging timber bridges. One of the reasons for such a large number of timber structures lies in the fact that Australia, for example, was blessed with vast supplies of durable and strong hardwood timber. However, a point has been reached in Australia and elsewhere, where large sawn timber is not available in sufficient quanties to continue to economically maintain these structures in the traditional manner. It is well known that the supply of large diameter logs is quickly diminishing as old growth forests disappear. Consequently the cost of timber has risen sharply. Over the past ten years, the cost of good quality large sawn hardwood timber has risen by about a factor of four.
From an environmental point of view, alternative methods and materials are needed to replace aging timbers or reduce the need for such timbers.
Because timber is dimensionally unstable (that is as the moisture content varies it swells or shrinks by as much as ten percent perpendicular to the grain) and is also biologically unstable if exposed to the elements, it has to date been considered as a temporary material in outdoor construction. Thus timber structures have been repaired in the traditional manner which involves replacing members as they degrade.
Also, because natural round timber is irregular in its natural form, it is sawn to produce a more uniform product. Up to sixty percent of the timber is lost at the mill and a material of inferior strength is produced because some load carrying fibres are severed and knots are exposed.
The invention, in one application, replaces large sawn timber with reinforced concrete and combines this in the preferred embodiment with natural round timber which is in reasonable supply and can be obtained from plantations and regrowth forests.